The invention relates to an improved rearview mirror suitable for vehicles, especially motor vehicles, and particularly to the type of rearview mirror that contains a plurality of interference layers between the substrate glass and the metallic layer.
It is known that suitable reflection layers for such rearview mirrors comprise coatings having a total reflection which is not excessive, but is at least 40% according to an EC guideline, and which further exhibit a marked blue reflection tint. Such reflection layers have glare-reducing properties, owing to: on the one hand, reduced total reflection, e.g,. in comparison with silver or aluminum, and, on the other hand, the fact that the headlight beam, because of its rather low color temperature, exhibits an emission spectrum shifted to the yellow range, which spectrum of light is reflected by such reflection layers less strongly than daylight. Such coatings (layers) can be applied both to the front surface and to the rear of a transparent substrate, the latter in most cases being a soda silica glass.
Thus, there are known rearview mirrors coated on the front surface, having a reflection layer consisting of three dielectric individual layers of about .lambda./4 thickness each (.lambda. about 450 nm), with the outside two layers exhibiting high refractive index, and the middle layer a low refractive index. Because of the light transmission of the dielectric layers, the rear surface of such a mirror is provided in most cases with a dark, absorbing coating (DE-AS 1036672, DE-AS 2449763).
Further, front surface mirrors are known, which, besides a metal layer with 55 to 85% total reflection lying directly on the substrate, also have a dielectric layer with high refractive index and about .lambda./2 thickness (.lambda. about 450 nm). By such an additional interference layer, the original neutral reflection tint of the metal layer is converted into a blue tint. Since the metal layer is largely opaque on the front surface, the abovementioned rear surface dark coating can be dispensed with in such a mirror (DE-OS 37 28 100).
Such front surface mirrors, as is known, have the advantage of a reflection absolutely free of a double image, but at the same time exhibit the drawback that the thin reflection coating, in most cases only a few hundred nm, is exposed unprotected to outside mechanical and chemical attacks. As a result, a premature wearing of this coating often occurs, and the mirrors become useless.
On the other hand, rearview mirrors, coated on the rear surface, with a blue reflection tint are also known, in the form of three different coating types.
In the first type, a dielectric layer with high refractive index and about .lambda./2 thickness (.lambda. about 450 nm) is located directly on the substrate rear surface. Next, a metal layer with 60 to 80% total reflection follows and then optionally another protective layer. Just as in the front surface mirror described above, here also the originally neutral reflection tint of the metal layer is converted by an additional interference layer into a blue tint. But in the rear surface mirror, the .lambda./2 layer is adjacent to the substrate glass and not adjacent to the air as in the above-described front surface mirror. As a result this interface loses optical efficiency so that such a rear surface mirror exhibits only a relatively weak blue reflection tint. Further, this blue tint is achieved only by a reduction of the reflection in the long-wave visible spectral region. Therefore, the total reflection of such a rear surface mirror is clearly decreased by the incorporation of a blue reflection tint, so that the requirement for at least 40% total reflection possibly can no longer be met. This drawback can be avoided only by the use of materials having an extremely high refractive index, greater than 2.5, for the production of the .lambda./2 layer. But such materials, as, e.g., zinc sulfide, are chemically not very stable and can be applied only by a rather complex vaporization process. The just described mirror type does have the advantage, however, that the metal layer, e.g., can consist of chemically resistant chromium so that a rear protective coating is not necessary, but the disadvantages of this mirror type are a faint blue reflection tint and a simultaneously low total reflection (DE-PS 3436016 Cl).
In the second type, a very thin, partially translucent metal layer is located directly on the substrate rear surface; next, a dielectric layer with about .lambda./4 thickness (.lambda. about 750 nm) follows: then a metal layer again follows, but which is relatively thick and exhibits greater than a 90% total reflection. The refractive index of the dielectric layer of this mirror type is of no importance since it functions merely to bring about only a certain distance between the two metal layers, between which the interference phenomena occur. The metal layer, just described, having a high total reflection consists either of silver or aluminum, but both metals, because of their slight chemical resistance, must be provided with protective layers. Even so, especially in the case of silver, such protective layers are often not sufficient, resulting in an eventual attack and deterioration of the metals. On the other hand, the use of a highly reflective metal in the case of the just described mirror type is absolutely necessary; otherwise, the total reflection of the layer system does not exceed 40%. While with this mirror type, it is thus advantageously possible to achieve a marked blue reflection tint with sufficient total reflection, there is still a considerable drawback in using a highly reflective metal sensitive to chemical attack such as, e.g., silver (DE-PS 34 36 011 Cl).
In the third type there is also provided between the substrate and a metal layer with 50 to 80% total reflection a system of three dielectric interference layers, but which consist only of materials with high as well as low refractive indices. Here a layer with high refractive index and about .lambda./4 thickness (.lambda. about 470 nm) is located directly on the substrate rear surface, then a layer with low refractive index and about .lambda./4 thickness follows and then again a layer follows with a high refractive index, but about .lambda./2 thickness. Finally, the previously mentioned metal layer follows. The disadvantage of this layer system is in the choice of materials with high refractive index for the layer resting on the substrate. As a result, it is hardly possible to achieve a blue reflection tint and at the same time a total reflection of greater than 40%. Further, such a layer system exhibits not only the desired reflection maximum at about 450 nm, but also has a very marked reflection minimum at about 650 nm, so that here the spectral reflection again increases above 650 nm. But generally, at an acute-angled view of the interference layer system, such reflection maxima and minima move to shorter wavelengths, and that means in the case of this layer system, an undesirable change of the reflection tint to the reddish (JP-OS 602 127 04).